CONTROL & MEASUREMENT Master and slave

Electronic differential pressure measurement with two process transmitters

Liquid level monitoring by measuring differential pressure reached a new dimension: a system with two process transmitters in a master-slave arrangement. Compared to conventional measurement solutions, this set-up has the advantage of easier installation, a reduced temperature effect and higher precision.

When it comes to monitoring liquid levels, the market offers several different methods to choose from. In applications involving closed tanks or similar vessels, the level is frequently determined using a differential pressure transmitter. This proven technique is suitable in many situations, especially if an immersed measuring instrument is out of the question, for example because a grinder is installed or owing to an aggressive substance in the vessel.

In order to monitor the level, the differential pressure must be determined between the liquid and gaseous phases inside the tank. To enable the level to be calculated exactly, the tank geometry (horizontal or vertical, various possible lids and bottoms) and the specific density of the medium must be taken into consideration in addition to the hydrostatic pressure.

Differential pressure transmitters have two 1/4” NPT process connections according to IEC 61518 arranged side by side. This design reflects the instruments’ original measuring task, namely to determine a pressure drop, say, in pipes or at filters or pumps. When measuring liquid levels, on the other hand, the distance between the measuring points for the pressures P1 and P2 is much larger and must therefore be bridged with the aid of capillaries. These have a limited volume to ensure that pressure is transmitted without any interference. Measurement solutions with this kind of connection to the measuring points are sensitive to temperature effects which impact on the results. In the worst case, severe temperature fluctuations could falsify the measured value.

If the application requires higher accuracy, this measurement method soon comes up against its limits. In the meantime, however, an alternative solution exists which reduces the negative effects of conventional measurements to a minimum by using electronic differential pressure measurements to monitor liquid levels. The measuring system developed by WIKA is based on two separate process transmitters – for instance CPT-2x or IPT-2x, which achieve accuracy equivalent to up to 0.05% of the set span – in different configurations. These two devices work as a master and slave and have a purely electrical connection to one another via a signal cable that is not susceptible to failure.

They communicate over an internal bus.

The slave is supplied with power via the master and parameterised either via the interfaces or on the display of the master device. It measures the pressure P1, for example the gas pressure in a closed tank, and transmits it to the master. The pressure P2, which in this case is the pressure in the liquid column at the bottom of the tank, is recorded by the master. The latter then calculates the pressure difference between P1 and P2. The volume in the tank is determined by the master as an indication of the level based on this difference and taking account of other key process parameters like the tank shape and the density of the medium. This value can be output to the control room and / or the display of the master device as either an analogue or a digital signal.

The advantage of this solution is not simply that it reduces the influence of temperature to a minimum and enables higher precision. Electronic differential pressure measurements also take less time to get up and running. By contrast, an upstream test run is essential for measuring arrangements with a differential pressure transmitter to make sure that the capillaries work efficiently, for instance by eliminating air pockets from the impulse and pressure lines.

It is also far less complicated to replace the instruments if differential pressure is measured electronically. Furthermore, only one of the two devices needs to be exchanged in the event of a malfunction; the other one can then work together with the replacement (master or slave) device. When differential pressure is measured using capillaries, a defect inevitably necessitates a complete replacement, even if a small kink in the pressure line is the sole problem.

The electronic differential pressure measurement method described here could theoretically also be realised using two pressure transmitters. However, this would entail installing a third component with the crucial calculation unit as well as a second voltage source and two pressure inputs in the measuring arrangement. The electronic components which are needed to process the information and calculate the differential pressure are already integrated in a process transmitter.

WIKA picture: DP01 SEPTEMBER/OCTOBER 2020 Numerous liquid level applications profit 13

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